<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>背景</title>
	<style>
		.box{
			width:100px;
			height:100px;
			/*background-color:red;*/
			/*background-color:rgb(255,0,0);*/
			/*background-color:#f00;*/

			/*最后一个是透明度,1是完全不透明,0是完全透明*/
			background-color:rgba(255,0,0,0.5);
		}
	</style>
</head>
<body>
	<div class="box">
		
	</div>
</body>
</html>